Bioplastics based on wheat gluten processed by extrusion

Depósito de investigación de la Universidad de Sevilla https://idus.us.es/ “This is an Accepted Manuscript of an article published by Elsevier in Journal of Cleaner Production on Jiménez‐Rosado, M., Zárate-Ramírez, L., Romero, A., Bengoechea, C., Partal, P., & Guerrero, A. (2019). Bioplastics based on wheat gluten processed by extrusion. Journal of Cleaner Production, 239, 117994. available at: https://doi.org/10.1016/j.jclepro.2019.117994 1 Bioplastics based on wheat gluten processed by extrusion 2 M. Jiménez-Rosadoa*, L.S. Zarate-Ramírezb, A. Romeroa, C. Bengoecheaa, P. Partalc, 3 A. Guerreroa 4 5 a Departamento de Ingeniería Química, Universidad de Sevilla, Escuela Politécnica Superior. 6 Calle Profesor García González 1, 41012 Sevilla Spain 7 b Escuela Colombiana de Ingeniería “Julio Garavito” AK.45 No.205-59, Bogotá, Colombia 8 c Departamento de Ingeniería Química, Química Física y Química Orgánica, Facultad de 9 Ciencias Experimentales, Universidad de Huelva, Avenida Tres de Marzo, s/n, 21071 Huelva, 10 Spain 11 12 *Corresponding autor: Mercedes Jiménez-Rosado. Departamento de Ingeniería 13 Química, Universidad de Sevilla, Escuela Politécnica Superior. Calle Virgen de África, 14 7. 41012 Sevilla Spain Tel.: +34 954 557 179; fax: +34 954 556 447; E-mail: 15 mjimenez4[email protected] 16 17 18 19 20 21 22 23 24 25 Journal: 26 Running title header: Influence of modification of protein-based biodegradable 27 polymeric materials processed through extrusion 28 29 ABSTRACT 30 Recently, bioplastic have generated an increasing interest as an alternative to 31 conventional plastics. For this reason, their manufacture using the traditional techniques 32 used for the production of plastics, such as extrusion, would help transferring 33 bioplastics production to an industrial scale. In this way, the preparation of wheat gluten 34 bioplastics by extrusion was the main objective of this research, modifying their 35 structure by varying the pH value or by incorporating additives (glyoxal or xanthan 36 gum). These bioplastics were characterized by the measurement of their mechanical 37 properties and their water uptake capacity, proving that the modification of bioplastics 38 cause variations in their properties. Thus, extrusion resulted in a greater gluten- 39 plasticizer compatibility compared to compression, as denoted the temperature ramp 40 tests, especially in the presence of additives (ie. Xanthan gum, glyoxal). Moreover, 41 tensile strength was enhanced at pH 9, probably due to bonding promotion at alkaline 42 conditions. These results demonstrate the great potential of these materials for the 43 replacement of conventional plastics. 44 45 Keywords: rheology; additives; wheat gluten; extrusion; bioplastics 46 1. Introduction 47 Bioplastics, such as protein-based bioplastics, are a potential alternative to replace 48 conventional plastics. So they would help minimizing the environmental issues 49 associated with the predominant use of plastics derived from petrochemicals (Derraik, 50 2002; Sheavly and Register, 2007). Moreover, it should be highlighted that the Food 51 and Agriculture Organization (FAO) has reported that around 1.3 billion tons of food 52 are lost or wasted every year in the world. In addition, the fact that the land area used to 53 grow biomass for the production of bioplastics in 2018 corresponded to less than 0.02 54 percent of the global agricultural area (European Bioplastics, 2018), supports the idea 55 that there is no competition between the use of biomass to produce bioplastics and the 56 use of biomass for food and feed. In this context, the use of wheat gluten, which is 57 obtained as a by-product from the bio-ethanol industry, to make protein-based 58 bioplastics is a potential option (Ye et al., 2006). Thus, Guillaume et al. (2010) showed 59 that wheat gluten-coated paper was very effective at improving the shelf life of 60 mushrooms compared to PVC films; Tunc et al. (2007) studied its properties as 61 nanocomposite and Gennadios et al. (1994) found that it is a very effective oxygen 62 barrier at low relative humidity. Moreover, it is used mainly as animal feed, thereby its 63 use as raw material for bioplastics could increase its value (Ye et al., 2006). Therefore, 64 its use in the development of bioplastics would result in a revalorization of an abundant 65 by-product of the food industry (Patni et al., 2014). In this way, as wheat gluten is 66 profusely available and relatively inexpensive, a growing interest on its potential 67 application as packaging for the food industry has arisen due to its interesting 68 biodegradation, film formation, gas barrier, and mechanical properties (Olabarrieta et 69 al., 2006). 70 One of the most used techniques for the production of plastics is extrusion, due to its 71 high mixing and molding efficiency for thermoplastic materials (Pommet et al., 2003). 72 Most of the commercial synthetic plastic films (i.e. polyethylene) are produced in 73 extruders (Hernandez-Izquierdo and Krochta, 2008). The use of this widespread 74 technology with an agricultural renewable raw material, such as wheat gluten, 75 represents an interesting opportunity for the industrial production of bioplastics (Gug et 76 al., 2018; Herniou-Julien et al., 2019; Mathiot et al., 2019; Pommet et al., 2003), 77 helping their industrial scale-up (Ferrar et al., 2018). Unlike thermoplastic polymers, 78 wheat gluten protein may undergo some crosslinking reactions upon heating, which 79 would eventually result in an increase in the viscoelastic properties of plasticized 80 gluten-based materials. These structural changes occur at lower temperatures compared 81 to synthetic polymers (Balaguer et al., 2014; Lagrain et al., 2010). In this way, the 82 extrusion of wheat gluten is only possible under certain operating conditions, ranging 83 from the point at which protein chains begin to flow to the point where they start to 84 aggregate. 85 In the extrusion of wheat gluten proteins, certain key parameters must be considered in 86 order to have optimal operating conditions, such as plasticizer content, mechanical 87 energy input, applied shear, as well as operating time, temperature and pressure. 88 Therefore, the degree of aggregation, molecular conformational changes and chemical 89 crosslinking that occur along the extrusion process can be determined by these 90 parameters. In this respect, it is important not only to select an adequate temperature 91 window to obtain a moderate viscosity, which facilitates the processing of the 92 protein/plasticizer blend, but also to ensure a proper aggregation of protein molecules 93 that allows the formation of a homogeneous material, without exceeding thermal 94 conditions leading to degradation. On the one hand, the lowest temperature is defined 95 by the glass transition temperature of the protein, which is required to allow the material 96 to achieve its final shape (Ullsten et al., 2009). On the other hand, the upper temperature 97 limit is defined by either an extensive aggregation or the thermal degradation of the 98 protein. Thus, wheat gluten materials are generally processed between 80 and 130 ºC, 99 with their properties depending on those operating conditions (Redl et al., 2003). 100 Another parameter to be considered when replacing conventional plastics with any 101 alternative bioplastic is their final properties, such as their mechanical characteristics 102 and water uptake capacity. Within this scope, several techniques have been investigated 103 to reinforce bioplastics and improve their characteristics (Rasheed et al., 2018; 104 Thammahiwes et al., 2018, 2017). In this manner, studies can be found where the pH of 105 the bioplastics has been modified (Cortes-Trivino and Martinez, 2018), altering the net 106 charge of the protein system as it moves further away from the isoelectric point. Thus, a 107 pH shift may enhance certain bioplastic properties, such as Young’s modulus or 108 absorption capacity, as it strongly influences the nature of intermolecular interactions. 109 Another procedure investigated to improve the quality of bioplastics is the inclusion of 110 different additives in the system (Cieśla et al., 2006; Erdohan and Turhan, 2005; 111 Gounga et al., 2007; Parris et al., 1995; Siracusa et al., 2008; Turkoz et al., 2018). The 112 addition of polysaccharides into a protein matrix, with both of them being complex 113 heteropolymers, may promote the formation of non-covalent interactions and 114 intermolecular linkages between them (Turgeon et al., 2003). For this reason, protein- 115 polysaccharide mixtures may present a wide range of structures with different 116 rheological and physical-chemical properties that may allow the tailoring of the final 117 properties of the bioplastic material (Coughlan et al., 2004; Gómez-Martínez et al., 118 2009; López-Castejón et al., 2016; Schmitt et al., 2009, 1998; Turgeon and Beaulieu, 119 2001; Zaleska et al., 2000). Although some gums, such as xanthan gum, have proved to 120 be stable over a wide pH and temperature range, as well as to enzymatic degradation or 121 in the presence of salts (Sworn, 2009; Vega et al., 2015), all these factors may alter the 122 protein-polysaccharide interactions. Since xanthan gum is an anionic polymer, 123 depending on whether the pH is below or above the protein isoelectric point (IEP), 124 attractive or repulsive electrostatic interactions between the protein and the 125 polysaccharide are expected (Gosh and Bandyopadhyay, 2012). On the other hand, the 126 incorporation of crosslinking additives, such as aldehydes (e.g. glyoxal), to wheat gluten 127 protein-based bioplastics has proved to promote covalent crosslinking both intra- and 128 intermolecularly in wheat gluten protein (Zárate-Ramírez et al., 2014b). However, 129 Bruyninckx et al. (2016) have recently shown that an increase in crosslink density does 130 not always lead to enhanced mechanical properties of gluten-based bioplastics. These 131 authors postulated that crosslinks between protein segments may lead to a positive 132 reinforcement of the protein network, although but they may also force some kind of 133 unfavorable configurations that could prevent the formation of further physical 134 entanglements, thus compromising the overall network quality. 135 Taking into account all these parameters, the overall aim of this research was to process 136 biodegradable protein-based bioplastics with wheat gluten by extrusion, comparing this 137 with the compression molding method and determining the influence of the pH 138 modification and the addition of xanthan gum (polysaccharide) and glyoxal (aldehyde) 139 to them. In this way, their rheological properties and water uptake capacity were 140 studied. 141 2. Experimental 142 2.1. Materials 143 The protein-based biopolymer used was wheat gluten (WG) from Productos Riba S.A. 144 (Spain). This material, according to its technical data sheet, contains a minimum of 83 145 wt.% protein, 10 wt.% starch, 3 wt.% lipids and 1 wt.% ashes and the rest is moisture. 146 Glycerol (GL), provided by Panreac Química S.A.U. (Spain), and distilled water (W) 147 were used as protein plasticizers. Xanthan gum (Xanthomonas Campestris, XG), from 148 Sigma Aldrich (USA) and glyoxal (GXAL) from Panreac Química S.A.U. (Spain), were 149 used as additives. Finally, NaOH 2 M solution was used to modify the pH of the 150 systems from 6 to 9. 151 2.2. Processing of gluten/plasticizer samples 152 2.2.1. Batch Mixing of gluten/plasticizer samples 153 Prior to extruding, gluten/glycerol/water blends were obtained by a mixing stage using a 154 mixer torque rheometer fitted with two delta-shaped counter-rotating rollers which turn 155 at an angular velocity ratio of 3:2 (Brabender Plastograph, PL 3s model). In the 156 literature on rheometry, a detailed description of this equipment, which can record the 157 evolution of torque over mixing time, may be found (Dealy, 1982). These dough-like 158 blends, which were referred to as reference, had a pH of 6 and were composed of 50 159 wt.% WG, 18 wt.% GL, 32 wt.% W, following the relation used previously (Zárate- 160 Ramírez et al., 2014a, 2014b, 2011). 161 Some modifications were made to this reference sample: (i) by adding the necessary 162 amount of NaOH 2M solution to reach a pH of 9; (ii) by adding 3 wt.% GXAL with 163 respect to the wheat gluten protein percentage or (iii) by adding 1.5 wt.% XG dispersed 164 in the water fraction. The percentage of XG or GXAL was counteracted with the 165 percentage of water required to maintain the initial ratio WG:GL (25:9). These 166 modifications were carried out following the same procedure used in previous papers 167 (Zárate-Ramírez et al., 2014b) (Zárate-Ramírez et al., 2014a) 168 The mixing process was carried out under adiabatic conditions at 50 rpm for 10 min 169 approximately. Other authors have also used this mixing step (Ottenhof and Farhat, 170 2004; Türe et al., 2012; Ullsten et al., 2009) in order to facilitate the thermomechanical 171 processing through extrusion (Chantapet et al., 2013; Ottenhof and Farhat, 2004), 172 thermo or compression molding (Jerez et al., 2007; Sun et al., 2008) and injection 173 molding (John et al., 1998). All of them, traditional polymer processing techniques that 174 can improve the industrial escalation of this bioplastic. However, the properties of the 175 final dough-like material blends may affect their subsequent processing. Thus, it is very 176 convenient to monitor parameter such as, torque and temperature throughout the mixing 177 time. 178 2.2.2. Extrusion of gluten/plasticizer bioplastics 179 The dough-like materials after the mixing step were introduced into a Rheomex 302p 180 screw extruder (Thermo-Haake, Germany) coupled to the mixer. This extruder has a 181 spindle barrel diameter of 19 mm with a total length of 830 mm. In addition, it has an 182 electric heating system at four zones (feed, nozzle and two in the mixing zone) from 183 where the temperature profile indicated in Table 1 was established for the different 184 blends (based on previous studies). The nozzle used was of laminar geometry, and the 185 spindle speed applied was 30 rpm. 186 attributed to a glass transition of the plasticized wheat gluten (Jerez et al., 2005; Sun et 303 al., 2008, 2007). The change from compression molding processing to extrusion causes 304 an approach between the two peaks. A further approach takes place when any of the 305 additives is incorporated into the bioplastic, suggesting a greater degree of compatibility 306 between phases in the presence of the additives included. Therefore, extrusion proves to 307 be a more effective processing technique for the production of gluten bioplastics, 308 regarding the compatibility among ingredients. Moreover, extrusion shows the 309 advantage of being a continuous process compared to the discontinuous compression 310 molding. 311 The high values of E’ for bioplastics with gum and the overlap of the tan δ peaks for 312 specimens with GXAL and XG, indicate that the shear and the relatively high 313 temperatures inside the extruder favour a greater degree of crosslinking and 314 compatibilization of the systems that incorporate these components, compared to those 315 that are free of additives. In fact, the shear can decrease the activation energy of the 316 crosslinking reactions and facilitate the interaction between the reactive groups of the 317 additives and those of the protein chains, while the high profiles of temperature would 318 help fix the structure of the bioplastic matrix (Domenek et al., 2003; Redl et al., 2003). 319 3.2.2. Uniaxial tension measurements 320 Figure 4 shows the values of the parameters obtained from tensile tests for the different 321 bioplastics studied. This figure collects the maximum strain (εmax), the maximum stress 322 (σmax) and the Young’s modulus (E) values. First, the change from compression 323 molding to extrusion processing produced an increase in εmax and E values, but a 324 decrease in σmax values. On the other hand, regarding the different bioplastics made by 325 extrusion, the WG/GL/W specimen prepared at pH 9 showed the highest values of σmax 326 and εmax. This behaviour may be due to the fact that the alkaline pH favours the 327 aggregation of wheat gluten proteins and the formation of bonds through SH-SS 328 exchange, oxidation of SH groups, lanthionine formation (LAN) and/or Maillard 329 reactions (Gerrard and Brown, 2002; Olabarrieta et al., 2006). The slightly darker 330 extrusion profiles obtained at pH 9 seems to support this later mechanism. Therefore, 331 the alkaline medium would allow the formation of a wheat gluten matrix with a good 332 degree of crosslinking, being more stable and with stronger intermolecular interactions 333 between components. Secondly, the high temperatures used for the formation of sheets 334 with GXAL and XG lead to a high polymerization, giving rise to bioplastics with low 335 values of σmax and εmax, which are more pronounced for the bioplastics containing 336 GXAL (WG/GL/GXAL-W). 337 338 Figure 4: Representative parameters (σmax, εmax and E) obtained from tensile strength tests performed on the different 339 samples processed by compression moulding and extrusion. WG: Wheat gluten. GL: Glycerol. W: Water. GXAL: 340 Glyoxal. XG: Xanthan gum. 341 Mechanical properties of wheat gluten bioplastics here presented are well below that 342 reported for some typical commercial polymeric materials, like polypropylene (E: 1.5-2 343 GPa) or epoxies (E: 2-3 GPa), (Lagrain et al., 2010) and even lower than low molecular 344 weight polyethylene (LDPE. E: 250 MPa) (“ASTM D638-14: Standard Test Method for 345 Tensile Properties of Plastics,” 2014) but bring biodegradability or a dependency on a 346 renewable resource as advantages. However, they still need to prove they are cheaper to 347 produce than their petrochemical counterparts to successfully penetrate in the plastic 348 market (Patni et al., 2014). 349 3.2.3. Water absorption tests 350 The parameters obtained from water absorption tests is showed in Figure 5. Thus, the 2 351 and 24 h water uptake capacity values and the soluble matter loss are represented. As 352 can be observed, the use of extrusion instead of compression molding lead to a clear 353 improvement for the water uptake capacity after 2h but to a reduction after 24 h. When 354 comparing the different bioplastics made by extrusion, the WG/GL/W system at pH 9 355 presented a noticeable improvement in water uptake (particularly after 24 h) over the 356 reference (WG/GL/W at pH 6 by extrusion), whereas the rest of specimens showed a 357 moderate decrease in values as compared to the reference, being the GXAL system 358 which yielded the lowest values. It is worth pointing out that among the extruded 359 systems, the specimen showing higher WUC was one of the systems processed under 360 milder thermal conditions and was also the system displaying the lowest Young’s 361 modulus. On the other hand, the bioplastics with additive (GXAL or XG) have a 362 crosslinked structure that would give less space to the interaction with water molecules, 363 resulting in lower percentages of water uptake capacity. 364 365 366 Figure 5: Water uptake capacity and soluble matter loss obtained for samples subjected to immersion for 2 and 24 h 367 of immersion. WG: Wheat gluten. GL: Glycerol. W: Water. GXAL: Glyoxal. XG: Xanthan gum. 368 As for the soluble matter loss, this was lower when the bioplastic was made by 369 compression molding. This fact together with the lower water uptake capacity denotes 370 that a more closed structure is achieved when the bioplastics are made through 371 compression molding. In addition, significant differences were not found between the 372 different systems elaborated by extrusion. 373 4. Conclusions 374 In order to face the environmental issues related to the ubiquitous presence of plastic 375 materials, it is interesting to assess the industrial production of bioplastic materials 376 through well-stablished technologies, such as extrusion or compression. 377 Firstly, it was observed that extrusion gave rise to bioplastics with greater compatibility 378 between their components when compared to compression molding, assuming an 379 improvement both in mechanical properties and water uptake capacity. These results 380 may be due to a greater level of aggregation/crosslinking and structural alignment 381 within the sheets in the flow direction of the extrusion. 382 Different results were obtained when the effect of pH or the presence of additives 383 (glyoxal and xanthan gum) were studied. Therefore, it was observed that a change of pH 384 to a more alkaline pH improved the water uptake capacity of the bioplastics. Although 385 this took place at the expense of mechanical properties such as Young’s modulus, the 386 physical integrity was clearly maintained. However, when bioplastics were produced 387 with additives (GXAL and XG), a higher polymerization occurred due to the high 388 temperatures used, giving rise to bioplastics with mechanical and absorption properties 389 which slightly differ from those of the reference system (WG/GL/W at pH 6 by 390 extrusion). 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